GameDevMarket.net
Great quality (but budget priced) 2D and 3D artwork you can use in your own games.
OGT Level Manager
Lock and unlock levels easily with OGT Level Manager for Corona SDK.
Game Development Crash Course
19 lessons and an hour of video get you started fast and free with Corona SDK game dev!
Super Sprite Bundle
Royalty-free graphics for 2D games - animated characters, tile sets, etc.
Mobile Game Dev Videos
8+ hours of video tutorials - see how to make games with Corona SDK!